A two component type developer for visualizing a negatively charged electrostatic latent image in electrophotography comprises a toner that is capable of being positively charged, and a positive-charge endowing i.e., negatively chargeable carrier.
Hitherto, an example of such a carrier is a single layer coated carrier wherein a carrier core material is coated with an acrylic resin.
The acrylic resin adheres well to the core material, and also exhibits good coating properties; however, the surface of carrier particles is readily contaminated with toner particles (that means poor resistance against contamination of the carrier surface). Furthermore, acrylic resin exhibits large negative-charge endowing ability.
In view of these drawbacks, use of a fluorinated polymer has been studied, and such a polymer is deemed a coating material exhibiting good resistance against surface-contamination and of which positive-charge endowing property relative to toner particles readily maintained.
Though having a strong positive-charge endowing property, the fluorinated polymer has disadvantages; for example, it poorly adheres to a core material; and pin holes are likely to be formed due to its poor coating property. Additionally, when using such a fluorine polymer coating material, flakes of the resin form lamination on the core material and the mechanical strength of individual particles is insufficient; flake-shaped resin peels off in the course of repeated use of a developer. In summary, the carrier coated with a fluorine polymer has been found unsatisfactory in long-range stability of a positive-charge endowing property.
Generally, a single layer-coated carrier should have a thicker coating layer to have a large charge capacity.
However, a thicker coating layer incurs various problems such as: a solvent used in forming a coating layer remains in the resultant coating layer; the fluidity of a developer is deteriorated; a level of charge potential does not readily rise.
Since a fluororesin excels in surface-fouling resistance, and is capable of endowing carrier particles with good powder properties, there have been studies as to the use of multi-layer coated carrier particles, with each particle comprising a plurality of coating layers.
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent 0.P.I. Publication) No. 110160/1987 discloses such a multi-layer coated carrier principally characterized in that individual particles comprise a core material provided thereon sequentially a triboelectricity controlling layer and a layer contains fluoroalkyl acrylate polymer or fluoroalkyl methacrylate polymer, with the respective monomeric units account for not more than 50 wt% of the surface layer material, wherein the surface layer is capable of being electrified depending on the polarity of the triboelectricity controlling layer.
In other words, this multi-layer coated carrier is a carrier for developing electrostatic latent image, and whose individual particles comprise a core material provided thereon sequentially a triboelectricity controlling layer (intermediate layer) and a self-cleaning type surface layer (outermost layer), and the content of fluoroalkyl (meth)acrylate polymer in the peeling-type surface layer (outermost layer) is not more than 50 wt%, whereby regardless of the characteristics of the resin of the peeling-type surface layer (outermost layer), it is possible to make the characteristics of the resin of the triboelectricity controlling layer (intermediate layer) prevail over the former, so as to vary the charge-endowing property of the carrier. Accordingly, the charge-endowing property of the carrier relative to the toner particles is always dependent upon the resin used in the triboelectricity controlling layer (intermediate layer).
If a positive-charge endowing resin is used as the resin of the triboelectricity controlling layer (intermediate layer), the charge-endowing property of the resin of the triboelectricity controlling layer (intermediate layer) becomes prevailing in the course of time. Correspondingly, the charge capacity of a developer in the initial stage of usage can be significantly different from that resulting from a developer undergone repeated use. Such a carrier has another problem; regardless of when a developer is used, whether at the initial stage of usage or after repeated use, the charge property on the carrier surface tends to be unstable, degrading the quality of developed images.
Japanese Patent 0.P.I. Publication No. 39880/1987 discloses a carrier for incorporation into a two-component developer, wherein individual carrier particles comprise a core material coated with an upper triboelectricity controlling layer and a lower triboelectricity controlling layer. This patent further discloses incorporation of a fluoroalkyl acrylate polymer in the upper triboelectricity controlling layer.
With this carrier, however, the charge-endowing property of the upper triboelectricity controlling layer is allowed to be prevailing at a comparatively early stage of service life, while the charge-endowing property of the lower triboelectricity controlling layer is allowed to develop far later (i.e. in the course of repeated use), in order to prevent excess accumulation of a charge potential caused by agitation of carrier. Therefore, it is impossible to provide a carrier that always exhibits a stable positive-charge endowing property. This type of carrier is disadvantageous in that under the conditions of a high temperature and high humidity, and depending upon the content ratio of the fluoroalkyl acrylate in the upper triboelectricity controlling layer, the property of resin in the lower triboelectricity controlling layer will prevailing on the surface of individual carrier particles, thus such a carrier does not serve as a carrier of stable positive-charge endowing property.
In summary, the charge-endowing property of the carrier described in the above-mentioned patent publication varies depending on a type of the resin of the triboelectricity controlling layer (intermediate layer), and is unsatisfactory as a carrier for use with the toner particles that incorporate, as a binder, a resin exhibiting a strong negative-charge endowing property, e.g. a polyester resin.